High energy solid formulations, such as propellants, explosives, and gasifiers, generally consist of particulate solids, such as fuel materials, oxidizers, or both, held together by an elastomeric binder. These high-energy formulations may also include a liquid plasticizer, such as a nitrate ester, which contributes to the elastomeric characteristic of the binder and adds additional energy to the formulation.
Conventional binders for high-energy formulations utilize cross-linked elastomers in which prepolymers are polyethers derived from oxetane compounds. Oxetanes can be substituted at the 3-position with methyl groups containing energetic functional groups such as nitro, nitrato, azido, and difluoroamino groups. Polyethers can be formed from these oxetane compounds by cationic polymerization. This technique employs an initiator formed from an adduct of an organic compound such as a diol (e.g., 1,4-butanediol) and a catalyst capable of causing cationic polymerization. Such catalysts include strong acids or Lewis acids. Once formed, the initiator reacts with one of the available oxetane monomers to form an initiating species, and polymerization proceeds by chain elongation until substantial, e.g., greater than about 95%, exhaustion of the monomers. Cationic polymerization of these oxetane compounds gives a hydroxy terminated polymer with a load bearing polyether backbone and pendant energetic groups.
Since these polymers are hydroxy-terminated, they are curable with isocyanates through chain extension and cross-linkable to form elastomers. Elastomers are formed from these polyethers by curing with isocyanates having a functionality of at least two, e.g., toluene diisocyanate. In order to obtain adequate mechanical properties, some degree of cross-linking of the polymer chains is required. Cross-linking is promoted by using an isocyanate of higher functionality or by adding a separate crosslinking agent, such as trimethylolethane or trimethylolpropane.
Despite the general teaching that these cross-linked elastomers are useful as binders in high-energy formulations, there are some important disadvantages to using them as binders. Cross-linked elastomers, for example, must be cast within a short period of time after addition of the curative. Additionally, disposal of a cast, crosslinked propellant formulation is very difficult, except by burning, which poses environmental problems and concerns. These cross-linked elastomers, therefore, are difficult to use in continuous processing, and they present a problem for removal during demilitarization.
In view of the inherent disadvantages of using cross-linked elastomeric polyethers as binders, attempts have been made to use oxetane monomers to prepare thermoplastic elastomers (TPEs) suitable for use as binders in high-energy formulations. High-energy oxetane monomers which are symmetrically di-substituted at the 3-position, such as 3,3-bis-(azidomethyl)oxetane (BAMO) or 3,3-bis-(nitratomethyl)oxetane (BNMO), give crystalline polymers known as hard blocks. Asymmetrically substituted oxetane monomers, such as 3-azidomethyl-3-methyloxetane (AMMO) or 3-nitratomethyl-3-methyloxetane (NMMO), give amorphous homopolymers known as soft blocks. Sequential polymerization of soft and hard block materials to give ABA block polymers by conventional methods have not given TPEs with useful properties. The use of low temperatures, dilute solutions, and bis-(cumyl chloride) catalysts have resulted in thermoplastic, elastomeric ABA block polymers with good mechanical properties suitable for use as binders in high-energy formulations, but these TPEs do not appear to be economically practical.
Star polymers contain several homopolymers attached at a single, central point and as a result, polymer chain-entanglement and crystallinity are radically altered. Star polymers and copolymers formed from oxetane compounds give materials having mechanical properties which make them useful as binders in high-energy formulations. Previous attempts to prepare star polymers by conventional multifunctional alcohol-boron trifluoride initiation, however, have not given materials with very useful properties.
In view of the foregoing, there exists a need for a novel method of preparing star polymers and polymer cascades having properties suitable for use as binders in high-energy formulations, such as propellants, gasifiers, or explosives.